The process of correcting distortion in an information-bearing signal is called equalization. This process may be performed on an information-bearing signal at passband or at baseband. The expression "passband" refers to the information-bearing signal prior to demodulation and the term "baseband" refers to the information-bearing signal after demodulation.
The typical equalizer structure is a time domain network wherein the product of a coefficient and the information signal at at least one predetermined time is formed to generate the equalizer output signal. In general, for this structure, the greater the number of coefficients, the more precise the distortion compensation. In addition, since the distortion in the transmission channel is time-varying, the coefficients must be adjusted so that the compensation provided varies with the distortion. A number of techniques having varying degrees of accuracy, such as zero forcing and least mean square, can be utilized to provide coefficient adjustment.
When the information-bearing signal takes the form of modulated quadrature-related carrier signals, four sets of coefficients are required for equalization at baseband. Each coefficient set includes one or more coefficients depending on the precision desired. Accordingly, four products at each predetermined time must be formed to produce the equalized baseband signals. Equalization of modulated quadrature-related carrier signals can be performed at passband using a technique disclosed in U.S. Pat. No. 3,755,738, issued Apr. 28, 1973 to R. D. Gitlin et al. This disclosed technique requires only two sets of coefficients. This requirement is also true of a refinement in the Gitlin et al approach disclosed in U.S. Pat. No. 4,247,940, issued Jan. 27, 1981 to K. H. Mueller et al. While both of these passband equalization techniques perform satisfactorily, their implementation is still rather complex as two products must be formed at each predetermined time and then combined into the equalizer output signal. As a result, the required circuitry can, at times, exceed system cost objects.